4.19-stable review patch. If anyone has any objections, please let me know.
------------------
From: Samuel Holland samuel@sholland.org
commit c950ca8c35eeb32224a63adc47e12f9e226da241 upstream.
The Allwinner A64 SoC is known[1] to have an unstable architectural timer, which manifests itself most obviously in the time jumping forward a multiple of 95 years[2][3]. This coincides with 2^56 cycles at a timer frequency of 24 MHz, implying that the time went slightly backward (and this was interpreted by the kernel as it jumping forward and wrapping around past the epoch).
Investigation revealed instability in the low bits of CNTVCT at the point a high bit rolls over. This leads to power-of-two cycle forward and backward jumps. (Testing shows that forward jumps are about twice as likely as backward jumps.) Since the counter value returns to normal after an indeterminate read, each "jump" really consists of both a forward and backward jump from the software perspective.
Unless the kernel is trapping CNTVCT reads, a userspace program is able to read the register in a loop faster than it changes. A test program running on all 4 CPU cores that reported jumps larger than 100 ms was run for 13.6 hours and reported the following:
Count | Event -------+--------------------------- 9940 | jumped backward 699ms 268 | jumped backward 1398ms 1 | jumped backward 2097ms 16020 | jumped forward 175ms 6443 | jumped forward 699ms 2976 | jumped forward 1398ms 9 | jumped forward 356516ms 9 | jumped forward 357215ms 4 | jumped forward 714430ms 1 | jumped forward 3578440ms
This works out to a jump larger than 100 ms about every 5.5 seconds on each CPU core.
The largest jump (almost an hour!) was the following sequence of reads: 0x0000007fffffffff → 0x00000093feffffff → 0x0000008000000000
Note that the middle bits don't necessarily all read as all zeroes or all ones during the anomalous behavior; however the low 10 bits checked by the function in this patch have never been observed with any other value.
Also note that smaller jumps are much more common, with backward jumps of 2048 (2^11) cycles observed over 400 times per second on each core. (Of course, this is partially explained by lower bits rolling over more frequently.) Any one of these could have caused the 95 year time skip.
Similar anomalies were observed while reading CNTPCT (after patching the kernel to allow reads from userspace). However, the CNTPCT jumps are much less frequent, and only small jumps were observed. The same program as before (except now reading CNTPCT) observed after 72 hours:
Count | Event -------+--------------------------- 17 | jumped backward 699ms 52 | jumped forward 175ms 2831 | jumped forward 699ms 5 | jumped forward 1398ms
Further investigation showed that the instability in CNTPCT/CNTVCT also affected the respective timer's TVAL register. The following values were observed immediately after writing CNVT_TVAL to 0x10000000:
CNTVCT | CNTV_TVAL | CNTV_CVAL | CNTV_TVAL Error --------------------+------------+--------------------+----------------- 0x000000d4a2d8bfff | 0x10003fff | 0x000000d4b2d8bfff | +0x00004000 0x000000d4a2d94000 | 0x0fffffff | 0x000000d4b2d97fff | -0x00004000 0x000000d4a2d97fff | 0x10003fff | 0x000000d4b2d97fff | +0x00004000 0x000000d4a2d9c000 | 0x0fffffff | 0x000000d4b2d9ffff | -0x00004000
The pattern of errors in CNTV_TVAL seemed to depend on exactly which value was written to it. For example, after writing 0x10101010:
CNTVCT | CNTV_TVAL | CNTV_CVAL | CNTV_TVAL Error --------------------+------------+--------------------+----------------- 0x000001ac3effffff | 0x1110100f | 0x000001ac4f10100f | +0x1000000 0x000001ac40000000 | 0x1010100f | 0x000001ac5110100f | -0x1000000 0x000001ac58ffffff | 0x1110100f | 0x000001ac6910100f | +0x1000000 0x000001ac66000000 | 0x1010100f | 0x000001ac7710100f | -0x1000000 0x000001ac6affffff | 0x1110100f | 0x000001ac7b10100f | +0x1000000 0x000001ac6e000000 | 0x1010100f | 0x000001ac7f10100f | -0x1000000
I was also twice able to reproduce the issue covered by Allwinner's workaround[4], that writing to TVAL sometimes fails, and both CVAL and TVAL are left with entirely bogus values. One was the following values:
CNTVCT | CNTV_TVAL | CNTV_CVAL --------------------+------------+-------------------------------------- 0x000000d4a2d6014c | 0x8fbd5721 | 0x000000d132935fff (615s in the past) Reviewed-by: Marc Zyngier marc.zyngier@arm.com
========================================================================
Because the CPU can read the CNTPCT/CNTVCT registers faster than they change, performing two reads of the register and comparing the high bits (like other workarounds) is not a workable solution. And because the timer can jump both forward and backward, no pair of reads can distinguish a good value from a bad one. The only way to guarantee a good value from consecutive reads would be to read _three_ times, and take the middle value only if the three values are 1) each unique and 2) increasing. This takes at minimum 3 counter cycles (125 ns), or more if an anomaly is detected.
However, since there is a distinct pattern to the bad values, we can optimize the common case (1022/1024 of the time) to a single read by simply ignoring values that match the error pattern. This still takes no more than 3 cycles in the worst case, and requires much less code. As an additional safety check, we still limit the loop iteration to the number of max-frequency (1.2 GHz) CPU cycles in three 24 MHz counter periods.
For the TVAL registers, the simple solution is to not use them. Instead, read or write the CVAL and calculate the TVAL value in software.
Although the manufacturer is aware of at least part of the erratum[4], there is no official name for it. For now, use the kernel-internal name "UNKNOWN1".
[1]: https://github.com/armbian/build/commit/a08cd6fe7ae9 [2]: https://forum.armbian.com/topic/3458-a64-datetime-clock-issue/ [3]: https://irclog.whitequark.org/linux-sunxi/2018-01-26 [4]: https://github.com/Allwinner-Homlet/H6-BSP4.9-linux/blob/master/drivers/cloc...
Acked-by: Maxime Ripard maxime.ripard@bootlin.com Tested-by: Andre Przywara andre.przywara@arm.com Signed-off-by: Samuel Holland samuel@sholland.org Cc: stable@vger.kernel.org Signed-off-by: Daniel Lezcano daniel.lezcano@linaro.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- Documentation/arm64/silicon-errata.txt | 2 + drivers/clocksource/Kconfig | 10 ++++++ drivers/clocksource/arm_arch_timer.c | 55 +++++++++++++++++++++++++++++++++ 3 files changed, 67 insertions(+)
--- a/Documentation/arm64/silicon-errata.txt +++ b/Documentation/arm64/silicon-errata.txt @@ -44,6 +44,8 @@ stable kernels.
| Implementor | Component | Erratum ID | Kconfig | +----------------+-----------------+-----------------+-----------------------------+ +| Allwinner | A64/R18 | UNKNOWN1 | SUN50I_ERRATUM_UNKNOWN1 | +| | | | | | ARM | Cortex-A53 | #826319 | ARM64_ERRATUM_826319 | | ARM | Cortex-A53 | #827319 | ARM64_ERRATUM_827319 | | ARM | Cortex-A53 | #824069 | ARM64_ERRATUM_824069 | --- a/drivers/clocksource/Kconfig +++ b/drivers/clocksource/Kconfig @@ -365,6 +365,16 @@ config ARM64_ERRATUM_858921 The workaround will be dynamically enabled when an affected core is detected.
+config SUN50I_ERRATUM_UNKNOWN1 + bool "Workaround for Allwinner A64 erratum UNKNOWN1" + default y + depends on ARM_ARCH_TIMER && ARM64 && ARCH_SUNXI + select ARM_ARCH_TIMER_OOL_WORKAROUND + help + This option enables a workaround for instability in the timer on + the Allwinner A64 SoC. The workaround will only be active if the + allwinner,erratum-unknown1 property is found in the timer node. + config ARM_GLOBAL_TIMER bool "Support for the ARM global timer" if COMPILE_TEST select TIMER_OF if OF --- a/drivers/clocksource/arm_arch_timer.c +++ b/drivers/clocksource/arm_arch_timer.c @@ -319,6 +319,48 @@ static u64 notrace arm64_858921_read_cnt } #endif
+#ifdef CONFIG_SUN50I_ERRATUM_UNKNOWN1 +/* + * The low bits of the counter registers are indeterminate while bit 10 or + * greater is rolling over. Since the counter value can jump both backward + * (7ff -> 000 -> 800) and forward (7ff -> fff -> 800), ignore register values + * with all ones or all zeros in the low bits. Bound the loop by the maximum + * number of CPU cycles in 3 consecutive 24 MHz counter periods. + */ +#define __sun50i_a64_read_reg(reg) ({ \ + u64 _val; \ + int _retries = 150; \ + \ + do { \ + _val = read_sysreg(reg); \ + _retries--; \ + } while (((_val + 1) & GENMASK(9, 0)) <= 1 && _retries); \ + \ + WARN_ON_ONCE(!_retries); \ + _val; \ +}) + +static u64 notrace sun50i_a64_read_cntpct_el0(void) +{ + return __sun50i_a64_read_reg(cntpct_el0); +} + +static u64 notrace sun50i_a64_read_cntvct_el0(void) +{ + return __sun50i_a64_read_reg(cntvct_el0); +} + +static u32 notrace sun50i_a64_read_cntp_tval_el0(void) +{ + return read_sysreg(cntp_cval_el0) - sun50i_a64_read_cntpct_el0(); +} + +static u32 notrace sun50i_a64_read_cntv_tval_el0(void) +{ + return read_sysreg(cntv_cval_el0) - sun50i_a64_read_cntvct_el0(); +} +#endif + #ifdef CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND DEFINE_PER_CPU(const struct arch_timer_erratum_workaround *, timer_unstable_counter_workaround); EXPORT_SYMBOL_GPL(timer_unstable_counter_workaround); @@ -408,6 +450,19 @@ static const struct arch_timer_erratum_w .read_cntvct_el0 = arm64_858921_read_cntvct_el0, }, #endif +#ifdef CONFIG_SUN50I_ERRATUM_UNKNOWN1 + { + .match_type = ate_match_dt, + .id = "allwinner,erratum-unknown1", + .desc = "Allwinner erratum UNKNOWN1", + .read_cntp_tval_el0 = sun50i_a64_read_cntp_tval_el0, + .read_cntv_tval_el0 = sun50i_a64_read_cntv_tval_el0, + .read_cntpct_el0 = sun50i_a64_read_cntpct_el0, + .read_cntvct_el0 = sun50i_a64_read_cntvct_el0, + .set_next_event_phys = erratum_set_next_event_tval_phys, + .set_next_event_virt = erratum_set_next_event_tval_virt, + }, +#endif };
typedef bool (*ate_match_fn_t)(const struct arch_timer_erratum_workaround *,